This invention relates generally to lasers, and more particularly, to laser resonators of cylindrical or annular configuration. Obtaining high-power beams from conventional linear laser resonators poses a number of difficult problems. These problems can be catagorized as relating to power extraction, beam quality and ease of fabrication. The output power of laser is proportional to the lasing medium volume. For most laser media, the lasing width is restricted to several centimeters. Thus, the other dimensions (length and height) must be large for high powers. An example of an implementation of a conventional linear resonator on a lasing medium configuration (large volume) for high powers is shown in FIG. 1, which is discussed in more detail below. The outcoupled beam from the configuration would have a height-to-width ratio of more than several hundred to one, and this would not match typical beam transfer optics and transmitting telescope configurations. Moreover, the fabrication and support of the tall and narrow optics required would be difficult. The laser resonator must have a cavity length that is either impracticably large, or has to be made in a folded configuration that increases the number of mirrors required. Long paths through the lasing medium can also result in degradation in beam quality, and such lasers are extremely sensitive to mirror alignment. In addition, the linear configuration would have a unidirectional flow of gases which would result in a net thrust on a laser. This would not be desirable for either airborne or space based applications.
For these and other reasons, designers of high-power lasers have more recently shifted their attention to cylindrical lasing medium and annular resonator configurations. Configuring the lasing medium in a cylindrical shape allows large lasing volumes to be realized with compactness and structural rigidity. There is also negation of flow thrust due to the radial symmetry. The remaining problem has been to find annular laser resonator configurations to extract the power from the lasing medium efficiently, while maintaining good beam quality.
The conventional linear laser with an unstable resonator has one important attribute. It provides inherently good mode control. Undesirable higher-order modes of operation of the laser are not present, and the laser therefore provides good beam quality. Cylindrical or annular lasers have problems in maintaining mode control and thus may not provide good beam quality. If some of the power could be sacrificed, spatial filtering could be employed to remove unwanted higher-order modes of lasing, but spatial filtering is inefficient from a power standpoint.
Various designs and proposals have been advanced to seek, in effect, the annular analog of the conventional unstable linear laser resonator. The ideal annular laser resonator configuration would be one that combined the advantage of beam quality, which is inherent in the unstable linear resonator, with the advantages of efficient use of area and thus high power, and of symmetry inherent in the annular configuration. However, as will be explained in more detail, annular laser configuration prior to this invention have been deficient in some important respects.
The common features of annular lasers are an annular gain region and an annular resonator. The principal requirement for the resonator is that it extract a large amount of power efficiently from the annular gain region, in such a manner that mode control, and therefore beam quality, are preserved.
The simplest annular resonator is the toric unstable resonator (TUR), which consists of two toric mirrors arranged at each end of the annular gain region. Since the toric optics have no single optic axis, there is no diffractive coupling in the azimuthal direction; thus there is no azimuthal mode control. Modifications to enhance mode control in the toric resonator have not been successful, and the configuration has been largely discarded by investigators.
An annular resonator configuration known as the half-symmetric unstable resonator with internal axicon (HSURIA) was intended to provide the desired combination of advantages. It combines the principal features of the toric unstable resonator, but also includes an optical element known as an axicon to convert the annular beam to a compacted cylindrical one. One form of the axicon is known as a waxicon, named for its letter-W shape when viewed in crosssection. A waxicon is basically an arrangement of two approximately conical mirrors. A first, outer conical mirror with an internal reflective surface reflects the annular beam inwardly toward a second, inner conical mirror, concentric with the first and having an external reflective surface. A section taken through a waxicon shows the two conical mirrors in a letter-W configuration. The annular beam is reflected radially in toward the optical axis of the waxicon by the first conical mirror, and is then reflected in an axial direction by the second conical mirror, the effect being to compact the annular beam into a cylindrical one, directed back along the central axis of the original annular beam. The compacted beam impinges on a scraper mirror, which reflects a central portion of the beam back into the resonator optics, and allows an out-coupled portion of the compacted beam to pass. Instead of a waxicon, a reflaxicon may be used. A reflaxicon also has two concentric conical mirrors, but the inner one is in a reversed orientation as compared with the waxicon. In a sectional view of a reflaxicon, the two conical mirrors appear to be parallel, and the compacted cylindrical beam continues in the same direction as the original annular beam.
The basic HSURIA configuration includes a waxicon or reflaxicon element at one end of the annular gain region and a plane mirror at the other end of the gain region. The resonator cavity is formed by the waxicon or reflaxicon, the plane mirror, and the feedback mirror, and has the simplicity of its toric optics and a single optical axis in the so-called "compact leg," in which the cylindrical beam is propagated. It is this single optic axis that gives this configuration some degree of mode control. However, the configuration also has some significant drawbacks.
Most importantly, the arrangement is extremely sensitive to mirror alignment, and particularly to any degree of tilt in the rear plane mirror. Substitution of a conic mirror for the plane mirror is sometimes made in an attempt to reduce this effect. The incident light in the annular beam is reflected from one side of the corner cube or conic reflector to the opposite side before being reflected back along the cavity. This poses a very serious polarization problem, in that the polarization of the light is scrambled by the conic or corner cube surface. A waxicon also inherently scrambles polarization, and it was ultimately discovered that the only physically allowable modes of operation of the HSURIA configuration were either radially or tangentially polarized. As a result, the light beam out-coupled from the resonator tends to be self-cancelling at the optical axis or center of the beam. Thus, the beam when focused and propagated over large distances would have a null on-axis and not produce the required high on-axis intensities.
One solution to the polarization problem is to coat the toric elements of the resonator with special phase-shifting coatings, such that no net polarization shift is produced in a round-trip passage through the resonator. However, the use of coatings tends to aggravate manufacturing problems, since the optical elements have to be made to an extremely fine tolerance. In particular, the apex of the inner conical surface of the waxicon or reflaxicon may not be truncated without losing mode control of the device and coating the surface near the tip is presently not feasible.
If the rear annular optical element of the HSURIA configuration is also replaced by a waxicon, the result is an annular ring resonator. The annular beam passes through the gain region, is compacted by one waxicon, passes through an aperture in a scraper mirror and is then expanded by the second waxicon. Although the annular ring resonator has some advantages over the basic HSURIA configuration, it provides only a single pass through the gain region, and is therefore less efficient in extracting energy from the gain region. Moreover, the annular ring configuration is very sensitive to the relative alignment of the two axicons (either waxicon pairs or reflaxicon pairs). This sensitivity is comparable to that of the HSURIA with a flat rear mirror element. An advantage of the annular ring resonator, however, is that polarization scrambling can be eliminated by the use of two axicons of the same type, i.e. two waxicons or two reflaxicons.
In a conventional annular ring resonator, the scraper mirror is also annular, having a central aperture to pass a return beam back to the compacted leg of the resonator. One approach that alleviates some of the problems of the annular ring resonator is referred to as decentered feedback. The scraper mirror has an offset aperture for passage of the return beam, and the result is that the optical axis of the resonator is no longer centered on the apex of the axicon. The apex can then be truncated without significant loss of mode control.
Even with decentered feedback, the annular ring resonator suffers from alignment sensitivity and low energy extraction efficiency. Accordingly, there is still a need for a resonator structure that overcomes these problems without sacrificing any of the advantages of the annular ring resonator. The present invention satisfies this need.